Fix drag-handoff bugs: gate FX re-resolve on outside-panel, cache unresolvable OS-drag verdict, block double FX-add retry

src/shell/panel/panel_state.h

@@ -320,6 +320,12 @@ struct PanelState {
     // when the pointer is not over an FX button.
     MediaTrack* instrumentDropTrack = nullptr;
 
+    // Latched once an OsDrag hand-off attempt for THIS gesture resolves to "cannot hand off"
+    // (empty/unresolvable payload, or the OS isn't ready) — skips re-running
+    // resolveDragPathsForOs (an fs::exists per sample) and the readiness probe on every
+    // subsequent move while the drag stays alive. Cleared by resetDragState.
+    bool dragOsHandoffBlocked = false;
+
     // Authoritative tail setting lives in ReaSamplerSession, not here; panel reads it for
     // drawing and mutates via footer click / scroll-wheel. bankPanelTailSetting is the
     // capture actions' read seam.